Impact of Soy and Low-Fat Dairy on Blood Pressure

Some earlier studies have suggested that eating a diet rich in low-fat dairy products can lower blood pressure.

The most well known of these studies was the Dietary Approaches to Stop Hypertension (DASH) diet study. It found that lowering sodium, limiting red meats and sweets, and eating two to three daily servings of low-fat dairy could reduce hypertension.

But other studies have not shown the association and some others have even suggested that a high-carbohydrate diet lowers blood pressure.

In an effort to better understand the impact, if any, of dietary protein and carbohydrates on blood pressure, Jiang He and colleagues conducted a study involving 352 patients with prehypertension or moderate hypertension (systolic blood pressure ranged from 120 to 159 and diastolic of 80 to 95).

Each study participant took powdered supplements of soy protein, milk protein, or refined carbohydrate in liquid twice a day for eight weeks. Milk and soy supplementation delivered 40 grams of protein daily, while the protein in the carbohydrate powder was negligible.

After a three-week "wash out" period the participants were put on another supplement for eight more weeks, followed by another three-week period without supplementation and another eight weeks on the third supplement.

Each participant took all three supplements over the course of the study, and since the powders looked and tasted the same, neither they nor the investigators knew which one they were taking at any given time.

The supplements were formulated to allow the researchers to compare the effects on hypertension without changing other dietary influences on blood pressure, including sodium, potassium, and calcium.

Blood pressure readings were taken three times at each of two clinical visits before and after each eight-week treatment phase.

Substituting Soy, Milk Protein for Carbohydrates

While no change in diastolic pressure was seen, milk protein supplementation was associated with an average lowering of 2.3 points in systolic blood pressure, and soy supplementation was associated with a 2.0 point reduction, compared to carbohydrate supplementation.

The study, which was funded by the National HeartLung and Blood Institute, appears in the Aug. 2 issue of Circulation.

Nutritionist and American Heart Association spokeswoman Rachel K. Johnson, PhD, MPH, tells WebMD that the study offers solid evidence that substituting soy or milk protein for carbohydrates can have a modest impact on blood pressure.

Johnson, who is a professor of nutrition at the University of Vermont and chair of the American Heart Association committee on nutrition, says one way to do this is to substitute soy milk or low-fat milk for sugar-sweetened beverages.

"We know that sugar-sweetened beverages are the No. 1 source of added sugars in the American diet, and several studies suggest that drinking fewer sugar-sweetened beverages can lower blood pressure," she says. "To me, this is just another nail in the coffin of these beverages."